The present invention relates to nutrient compositions for mammals such as infusions and sports drink and more particularly to nutrient compositions comprising L-glutamyl-L-cystine (hereinafter referred to as Glu-Cys-Cys) and/or L-glutamyl-L-cysteine disulfide [hereinafter referred to as (Glu-Cys).sub.2 ].
L-cysteine (hereinafter referred to as cysteine) is not considered to be an essential amino acid since cysteine is synthesized from L-methionine (hereinafter referred to as methionine) in a living body. Therefore, nutrient supplementation of sulfur-containing amino acids such as cysteine has been made using methionine. In recent years, however, it has been revealed that conversion of methionine into cysteine is not sufficiently made in bodies of newborns and patients with hepatic cirrhosis, homocystinemia, etc. It has thus been pointed out that for nutrient supplementation of sulfur-containing amino acids, it is necessary to supplement not only methionine but also cysteine [Metabolism, 37, No. 8, 796 (1988)].
It has further been revealed that demand for sulfur-containing amino acids increases after immoderate exercise or upon drinking and effectiveness of administration of cysteine or L-glutathione (.gamma.-L-glutamyl-L-cysteinyl-glycine, hereinafter referred to as glutathione) which has a cysteine residue has been recognized.
Cysteine is unstable in solution, so that it is difficult to add cysteine directly to a liquid nutrient composition such as an infusion. Use of N-acetyl-L-cysteine in place of cysteine has been studied and has been partly put into practice. However, it has been pointed out that there is a problem in its stability. Furthermore, glutathione having a cysteine residue is more stable than cysteine, but the problem remains unsolved when it is sterilized with heating. On the other hand, L-cystine (hereinafter referred to as cystine) is considered to be equivalent to cysteine from a nutriological standpoint, and it is extremely stable as the oxidized form of cysteine. However, because of low solubility in water (less than 0.11 g/l at 25.degree. C.), cystine cannot be used in an infusion, etc. Recently, glutathione disulfide having a cystine residue (oxidized form of glutathione) has been examined for this purpose but has not been provided for practical use yet.
As a method for improving the stability of cysteine, there is known a method in which cysteine is acetylated as described above. The method has been partly used practically in the prescription of infusion. With respect to cystine considered to be equivalent to cysteine from a nutriological standpoint, methods for improving the solubility by using glutathione disulfide having a cystine residue and by converting cystine into a peptide have been studied. As nutrient compositions containing cystine-containing peptides, there are known a composition comprising N,N'-bis-.alpha.-aspartyl-L-cystine [hereinafter referred to as (Asp-Cys).sub.2 ] (Japanese Published Unexamined Patent Application No. 151156/87), a composition comprising N.sup.2 -cysteinyl-N.sup.6 -L-cysteinyl-L-lysine [hereinafter referred to as Cys-Lys(Cys)] (DE 3206810), a composition comprising compounds represented by (X-Cys).sub.2 (wherein X is Gly, Ala, Leu, Ile or Phe) (EP 264953), a composition comprising bis(acetylglycyl)-L,L-cystine (Japanese Published Unexamined Patent Application No. 233999/88), and the like. Further, there are a report on the solubility of L-cysteinyl-bis-L-alanine [hereinafter referred to as (Cys-Ala).sub.2 ] in Clin. Nutr., Spec. Suppl. 4, 116-123 (1985) and a report on the behavior of bis-.alpha.-L-alanyl-L-cystine [hereinafter referred to as (Ala-Cys).sub.2 ] and bis-glycyl-L-cystine [hereinafter referred to as (Gly-Cys).sub.2)] in blood in J. Nutrition 118, 1470 (1988).
It has been desired to develop a technique for supplying unstable cysteine or cystine which is nutriologically equivalent to cysteine as nutrient compositions in a liquid form prepared by sterilization with heating.